Maybe instead of doing 28 straight, do 20, hang on the bar to take a breather and see if you can knock out another 10 or more after that rest. I'm assuming that you do the 28 straight through with no bar rest time. It still counts.
Or else , since muscular endurance is what you are training and what is useful to you train more reps with lighter weights, Say sets of 15 to 20 with 15 lbs.